Method of sherardizing



Patented Get. 28, 1%24.

' UNETE- C 1 PATENT FFHCE.

- OTHO v. STEWART, OF-WILKINSBURG, AND LEON MCCULLOCH, or PITTSBURGH, PENN- SYLVANIA, ASSIGNQRS T WESTINGHOUSE ELECTRIC AND MANUFACTURING COM- PANY, A CORPORATION OF PENNSYLVANIA.

METHOD OF SHERARDIZING.

No Drawing.

To all whom it may concern.

Be it known that we, OTHO V. STEWART,

a citizen of the United States, and a resident of Wilkinsburg, in the county. of Allegheny and State of Pennsylvania, and LEON MoCULLooH, a citizen of the United States, and a resident of Pittsburgh, in the county of Allegheny and State of Pennsylvania, have invented a new .and useful Improvement in Methods of Sherardizing, of which the following is a specification.

This invention relates to a methodof coating metal articles with zinc, commonly known as sherardizing, in order to render such articles more resistant to corrosion when exposed to various atmospheric conditions. I

Metal articles, such as-iron, whenexposed to the atmosphere, corrode very rapidly and, in a short time, become so deteriorated as to be useless. In order to prevent such deterioration, various protective coverings for such articles have been proposed. Among such coatings is one of zinc which is applied to the article at an elevated temperature. This method of coating is commonly known as sherardizing.

Heretofore, several methods of sherardizing to provide coatings of the above-indicated character have been proposed. Among these was one which consisted in heating the article to be coated, usually of iron, in contact with zinc dust containing 80% to 92% of powdered metallic zinc. The article to be coated was tumbled in a closed drum for a suitable length of time and heated to a temperature of 350 to 375 C. Such la procedure gave a fairly satisfactory coating which was resistant to corrosion by the atmosphere.

However, in practising this method, it is necessary that the zinc dust shall be substantially pure, containing substantially no impurities in addition to the zinc oxide which is mixed with the metallic zinc. The necessity for substantial u'rity of material adds to the expense 0 pre aration of such material and increases the ifliculty of obtaining the same. The temperatures heretofore employed have been limited to a very narrow range and it has been necessary to carefully adjust the furnace to keep the temperature within the very narrow limits.

specified. The cost of operation of the proc- Application filed Jully 9, 1921. Serial n0. 483,626.

ess was materially increased, owing to the close supervision necessary thereover.

Our invention is designed to obviate. the difliculties encountered in prior methods, it being among the objects thereof to provide a method of sherardizing which shall produce a heavier and more resistant coating than was produced by prior methods and one which is capable of resisting varying atmospheric conditions.

After a considerable amount of research, we have determined that several factors are of considerable importance in the development of a good sherardizing process which will result in a coating which is smooth, firm and adherent, and which is resistant to corrosion, such as is ordinarly produced by salt spray. We have discovered that the metallic-zinc content of the dust is an important factor in the process.

We have further. discovered that, the

temperature in the presence of zinc dust,

containing a small percentage of iron. We have found that 3%- to 10% of iron in'the zinc dust is beneficial to the sherardizing process. At the same time, the temperature of sherardizing may be raised to apoint considerably higher than had previously been used. For instance, zinc dust containing up to 7%.iron may be used for sherardizing articles at'temperatures up to about 430 C.,. thus producing heavier and much more resistant coatings-upon the article.

With higher percentages of iron, the temperature at which good coatings ma be obtained rises very rapidly. If the zinc dust contains as much as 10% iron, a temperature of 640 C. may be used without any tendency to fuse the dust. iron or in which iron is lackin good deposits cannot be obtained at t ese higher temperatures on account of the tendency of the dust to fuse. Generally, we utilize less than 7% of iron, although wemay utilize With zinc dust low in as much as and usually we prefer to confine ourselves to between 5% and 7% of higher temperatures named.

Tu accordance. with our discoveries, we prefer to heat the article to be sherardized to a, temperature ranging between 375 and d50 G. with zinc dust containing 7% iron or less. We generally utilize a temperature of 400 to 430 C. and a zinc dust containing 5% to 7% ironand to metallic zinc for the production of excellent coatings. The articles to be sherardized are'placed in contact with zinc dust containing iron in a closed drum which is revolved in a suitable furnace, preterably an electrically heated furnace. The heating and revolving are continued for a considerable length of time, usually about six hours. As a result, we obtain coatings containing about .18 gram zinc per square centimeter. I

Articles so coated were tested by exposure to salt spray for over 400 hours after which the coatings were found to be practically intact. Articles sherardized for. a similar length of time by one of the prior methods showed that .10 to .14- gram of zinc was deposited per square centimeter of surface, and, under the salt spray test, such coatings were brokenafter 110 to 130 hours.

A further set of comparative tests on materials made in accordance with our invention and of prior methods confirmed this result. Tron articles sherardized at 420 C. in a zinc dust containing 73% metallic zinc and 5% iron, were placed under a salt spray. The coatings were broken only after 404 hours exposure. A similar article made in accordance with the method of the prior art above set forth, when exposed to the salt spray test, failed after only 57 hours.

lit will be seen, therefore, that our improved process results in an article which is more heavily coated and which is much more resistant to corrosion than coatings produced by prior processes.

lln producing our zinc dust, it is not necesalmanac sary to utilize an expensive method, such as was used in the method of the prior art above set forth. We take zincdross, the waste obtained from hot galvanizing processes, and grind the same to a suitable fineness in the presence of heat. After the dust has been used a considerable length of time, the iron content rises to above the optimum limits, and the metallic zinc content is usually below such limits. The dust is then rejuvenated by the addition of pure metallic zinc dust. I

Although we have, in the above description, stated that certain temperatures and zinc dust of certain compositions give good results, it is to be understood that our invention is not limited to the details described,since it is obvious that our invention may be practiced at temperatures other than those specifically set forth and with zinc dust of varying grades of purity. We find that it is essential to have the zinc and iron content of the zinc dust within certain limits and that it is desirable to utilize a temperaaure corresponding to the composition of the ust.

We claim as our invention:

1. A method of coating metal which comprises heating the same for a predetermined length of time in the presence of zinc dust containing substantial amounts of iron, the temperature being substantially proportional to the iron content of the dust.

2.- A method of coating metal which comprises heatingthe same for a predetermined length of time in the presence of zinc dust containing-less than metallic zinc and less than 7% iron, at a temperature above 375 C.

3. A method of coating metal which comprises heating the same for a predetermined length of time in the presence of zinc dust containing 65% to 75% metallic zinc and 3% to 7% iron, at a temperature between 375 and 450 C. I

. 4. A method of coating metal which comprises heating the same. for a predetermined length of time in the presence of zinc dust containing about 70% metallic zinc and about 5% iron, at about 420 C.

In testimony whereof, we have hereunto subscribed our names this 2nd day of July,

@THU V. STEWART. LEQN MOGTHJLOGH. 

